It is known to provide a power steering system for a vehicle such as a motor vehicle to assist a driver in steering the motor vehicle. Typically, the power steering system is of a hydraulic type. The hydraulic power steering system employs an engine driven hydraulic power steering pump for generating pressurized fluid that is coupled to a hydraulic steering gear of the motor vehicle. Since the power steering pump is driven directly by the engine using a belt, its speed is determined by that of the engine and it operates continuously as long as the engine is running, resulting in continuous losses due to constant circulation of the hydraulic fluid through the steering gear. This results in increased fuel consumption due to continuous operation of the power steering pump at speeds dictated by the engine speed, even when no steering assist is required. In addition, the power steering pump has to provide the required flow and pressure for the worst-case engine speed, which could be near idle, under static steering conditions, contributing to significant fuel consumption. This results in much higher pump flow at higher engine speeds further increasing the losses in the hydraulic power steering system, which results ultimately in increased fuel consumption.
More recently, electrohydraulic power steering systems have been used to decouple the power steering pump form the engine and provide an on-demand hydraulic pressure using an electric motor to drive the hydraulic power steering pump. An example of such an electrohydraulic power steering system incorporates a hydraulic power steering pump driven by a brushless direct current electric motor controlled by a pulse width modulated inverter. Also, there are electrically driven steering systems without using any hydraulic fluids. Both of them improve the vehicle fuel economy by providing an on-demand steering power assist and minimizing/eliminating parasitic losses associated with the engine-driven hydraulic pump. However, the electrohydraulic power steering system needs a costly high power electric motor, power electronics for controlling speed of the electric motor, and a reliable electrical power supply (including an engine driven alternator and battery). The overall losses of the conventional electrohydraulic power steering system include losses through the engine alternator, power electronics, electric motor, and power steering pump.
As a result, it is desirable to provide a power steering system for a vehicle that minimizes power losses in a power steering pump and provides variable flow and pressure of the pump independent of engine speed. It is also desirable to provide a power steering system for a vehicle that controls pressure of the steering system using a power steering pump. It is further desirable to provide a power steering system that provides variable power steering assist at a relatively low cost. Therefore, there is a need in the art to provide a power steering system that meets these desires.